The field of this invention relates to a voltage regulator, an envelope tracking power supply system, a transmitter module and an integrated circuit device therefor.
In the field of advanced wireless transmitters, a technique known as envelope tracking can be used to improve power efficiency. With envelope tracking, the supply voltage to the power amplifier (PA) of the wireless transmitter is caused to approximately track the envelope of the transmitted radio-frequency (RF) signal. Thus, since the power dissipation in the PA is proportional to the difference between its supply voltage and output voltage, envelope tracking enables a reduction of power dissipation, and thereby power consumption, whilst allowing the PA to produce the intended radio frequency (RF) output.
This power reduction is possible if a switching regulator, such as a buck converter, is used to efficiently supply power to the PA. However, it is difficult to realize a buck converter that can modulate its output at a rate that is fast enough to match the envelope of a modern wireless communication system. Research has shown that a switching regulator arrangement comprising a buck converter in parallel with a Class-AB amplifier can provide power savings while tracking fast enough to support high data rate modulation schemes. In such a scheme, the Class-AB amplifier needs to be fast enough and powerful enough to cancel the ripple current that is output from the buck converter, as well as providing the high-frequency modulation. However, such a scheme comprising a buck converter is limited to providing the PA with a voltage that is less than that of the input power supply. In advanced wireless transmitters, the PA sometimes requires a voltage supply that is higher than that of the input power supply, typically provided by a lithium-ion battery.
There is therefore a need for an efficient voltage regulator that can step voltage either ‘up’ or ‘down’, and be connected in parallel with a Class-AB amplifier in order to track voltage fluctuations fast enough to support high-data-rate modulation schemes in an envelope tracking system.
A non-inverting buck-boost topology is not suitable for this application because its output current (the current that is integrated on its output capacitor) is not continuous. The Class-AB amplifier would therefore need to cancel a large alternating current at the switching frequency, with a magnitude larger than the load current.
A canonical single-inductor dc-dc converter does not have the ability to step up or down in a non-inverting fashion and also does not have a continuous output current.
An inverse-SEPIC (Single-Ended Primary-Inductor Converter), which uses two inductors, does have a continuous current output, and does have the ability to step up/down in a non-inverting fashion. However, this type of converter has the significant drawback that the voltage across some of the switches in the off state is twice that of the input supply voltage. For a power management integrated circuit (PMIC), this is a significant drawback because the voltage tolerance is usually chosen to be roughly equal to the input supply voltage.
Thus, a need exists for an improved voltage regulator apparatus for use within an envelope tracking power supply system.